Chapter 3: Muscle Structure and Function

Introduction

The skeletal muscles, like the joints, are designed to contribute to the body’s need for both mobility and stability. Muscles serve a mobility function by producing or controlling the movement of a bony lever around a joint axis; they serve a stability function by constraining extraneous movement of joint surfaces and through approximation of joint segments. Without muscle function, the body is incapable of either supporting itself against gravity or producing motion.

Human movement is a complex interaction of muscle function and joint lever systems under the control of the nervous system. Clinicians routinely evaluate the muscle function of patients to determine its status and to formulate appropriate interventions to help the patient regain or enhance function. Understanding muscle function begins with a clear picture of the muscle’s structure, from the contractile proteins within each muscle fiber to the organization of the fibers in the entire muscle. After examining muscle structure, we examine the basic mechanical properties of muscle fibers, the muscle in its entirety, and groups of muscles. To complete our understanding of muscle function, we will analyze the function of muscles working across joints to attempt to produce the intricate movements we use for daily activities, work, sport, and play. Patient applications will be used to apply principles related to basic muscle structure and function to selected common muscle injuries.

Elements of Muscle Structure

Skeletal muscles are composed of muscle tissue (contractile) and connective tissue (noncontractile). Muscle tissue has the ability to develop tension in response to chemical, electrical, or mechanical stimuli. Connective tissue elements, on the other hand, develop tension in response to passive loading.¹ Properties of the contractile and noncontractile tissues and the ways in which they are interrelated give muscles their unique characteristics.

Composition of a Muscle Fiber

Contractile Proteins

A skeletal muscle is composed of many thousands of muscle fibers that are grouped into fascicles. Each muscle fiber in a fascicle is composed of numerous myofibrils, and each myofibril is composed of numerous stacked myofilaments (Fig. 3–1). The muscle also includes a number of different connective tissue elements that we will return to later. The arrangement, number, size, and type of the muscle fibers may vary from muscle to muscle, but every fiber is a single muscle cell that is enclosed in a cell membrane called the sarcolemma.^2–4 Like other cells in the body, the muscle fiber is composed of cytoplasm, which in a muscle is called sarcoplasm. The sarcoplasm of the cell (fiber) contains myofibrils that are the contractile structures of the muscle fiber as well as non-myofibrillar structures such as ribosomes, glycogen, and mitochondria that are required for cell metabolism.